We'll be traveling to the north shores of Oahu and Kauai in late April / early May. What sort of surf conditions can I expect? I was there in late August a couple years ago and surf conditions in Hanalei were non-existent.

You can get some surf, just don't expect world class waves at that time of the year. I've seen Hanalei break decently in spring and I've seen it small/flat. Either way, it is one of the most picturesque places to surf in the world. I remember counting 32 waterfalls coming down Mt. Waialealea while sitting in the lineup. If Hanalei is breaking, then Tunnels is probably breaking also, a better break, IMO.
Your best bet on Oahu's North Shore is Sunset Beach, it picks up all kinds of swell. When the rest of the North Shore is small/flat, Sunset still breaks. It can be flat also, it all depends on the swell direction. Watch out for the shallow sections inside Backyards. If it is flat and you are looking for something to do, snorkle at Three Tables Beach, adjacent to Waimea Bay.